1. Field of the Invention
This invention relates generally to network interconnection of multiple electronic and consumer devices, and in particular to network interconnection for synchronized transmission of digital media to multiple devices.
2. Description of Related Art
Interconnecting Devices for Distribution of Audio, Telephone, Video, and Other Media Data
Conventional networking technology has not achieved practical home networking of consumer electronic devices. One fundamental obstacle to such networking is the difficulty of interconnecting various types of consumer electronics devices within a single room, or throughout an entire home or other environment, such as a hotel, apartment building, car, boat, or recreational vehicle. Moreover, when such devices are interconnected, there remain problems in distributing audio, video, and other types of media/data, in a sufficiently synchronized manner.
The advent of digital media and services (e.g., audio compact disks (CDs), direct satellite service (DSS) or digital video broadcast (DVB) from digital satellite broadcasts, and digital video disk (DVD) movies) increases the need for a synchronized network solution. Current systems for interconnecting consumer electronics and other devices have inadequate media synchronization, which is problematic. Synchronization is a major problem since delays among different devices in receiving media data can be on the order of hundreds of milliseconds, thereby resulting in distorted playback of content.
A particularly important network parameter is the reliability of on-time delivery of time-sensitive data, sometimes called “quality of service.” Quality of service in many prior-art networks is low, because many network protocols are collision-based (i.e., there is no handshaking or allocation of network resources between devices to preclude the need for repeat transmissions of time-sensitive data on congested networks). Audio, telephone, video, and other media are examples relying on the use of time-sensitive data, and any network to distribute time-sensitive data must have a high quality of service in order to be practical. The timing of media data, such as audio data, needs to be accurate to much less than one millisecond to avoid annoying phase and time synchronization problems between different audio speakers, or between audio speakers and video systems.
For example, speakers designed to play over a network connection (e.g., HomePlug, HomePNA, Ethernet, wireless, or equivalents) have a major problem with synchronization of playback. Traditionally, speakers are connected directly to an amplifier, which drives all of the speakers in the room. As long as the polarity of the connection to each speaker is correct, the amplifier controls the phase of the output from the speakers. This is possible, since there is effectively no latency from the time the amplifier produces a signal on the speaker wire to the time that signal arrives at the speaker. This is not true for networked speakers, due to a variable latency in the transmission of the audio data from the audio controller (the network stereo equivalent to a pre-amplifier/controller) to the speakers.
Variable latency causes the phase of the playback of the audio data to be different for each speaker in the system. One noticeable result of the phase difference is the degradation of the “surround sound” effects that consumers desire in a home theater system. Variable latency will also degrade stereo separation in a two-speaker configuration. Since home stereos are beginning to support 24-bit pulse code modulation (PCM) with a 96-kilohertz (kHz) sampling rate, improved synchronization will be needed to maintain acceptable playback quality.
One goal of networked speakers is to eliminate the speaker wires connecting each speaker to the central amplifier. Instead, each speaker has an internal amplifier and a network connection. If this network connection is power line based (e.g., HomePlug or HomePNA), then only a single connection to the speaker is required. Furthermore, most homes have many power outlets, making it easy to locate the speakers as needed.
Conventional methods attempt to minimize synchronization errors between networked speakers. One conventional method uses multicast transmission synchronized to the time of the first packet transmission. Another conventional method uses a network time protocol (NTP). Yet, both methods are problematic for the reasons discussed below.
The first method, multicast transmission, entails synchronizing playback of the audio content to the starting packet and is the simplest method. A controller uses multicast transmissions of packets, wherein each packet contains data for all the channels in the system. Each speaker starts playback with the first packet, and packets that are additionally received are buffered until the playback of earlier packets is completed. This method is relatively simple since it only requires a network interface card (NIC), a digital-to-analog (D/A) converter, and an amplifier in each speaker. However, this method has several problems. It requires that the clocks in all of the speakers be very accurate, or else the timing of the speakers will drift apart. If a speaker loses the network connection (e.g., is accidentally unplugged for an instant), the speaker will lose synchronization. HomePNA and some HomePlug proposals have link layer protocols (e.g., LARQ) that can cause variability in the arrival time of the initial packet. Therefore, although network synchronization is simple to implement by using the arrival time of the first packet, the multicast transmission method is not practical for high quality playback.
The second method, NTP, attempts to synchronize the clocks on the speakers and on the controller. NTP is a standard network protocol used to synchronize computer clocks to Universal Standard Time (UST); and is very accurate for time keeping, since clocks on computers coupled in a local area network (LAN) may be synchronized to about one millisecond accuracy. Unfortunately, such synchronization is not accurate enough for audio samples based on 24-bit samples taken with 96 kHz sampling rates. For example, transmission of 96 kHz audio samples would require about two orders of magnitude better speaker synchronization accuracy (i.e., 1/96,000 second or better).
Home Multimedia Networks
Current home multimedia networks (e.g., home theater systems) illustrate many of the fundamental problems discussed above. For example, home theater systems effectively are limited to a single room in a house, primarily due to the difficulty and expense of interconnecting devices. Each source device, such as a satellite receiver, VCR, laserdisc, or DVD player, typically is connected, by audio and video cables, to a “central” preamplifier or other form of switching device, which is also connected to a main television/monitor as well as to one or more power amplifiers. These power amplifiers are also connected to various speakers throughout the room by dedicated speaker cables.
Current conventional attempts to address the multi-room problem (i.e., “home multimedia networks”) exhibit a number of fundamental design flaws. For example, virtually all prior-art home network standards (e.g., X10, CEBus, Echelon LONWorks, and so forth) emphasize distribution of control information, but not audio, telephone, video, and other media. In other words, such systems distribute analog media on one network or wiring infrastructure, and distribute digital control information on another separate network. Custom installation of dedicated audio, telephone, video, and speaker cables throughout a home is required, often is prohibitively expensive, and results in relatively poor quality due to the degradation of analog signals propagating through multiple devices and extremely long cable runs.
The quality of such home automation networks is far below audiophile and videophile standards. Analog audio and video signals simply cannot propagate for long distances, and through multiple analog-to-digital (A/D) and digital-to-analog (D/A) converters and other analog processing circuitry, without suffering significant degradation in quality. Transmitting multiple audio/video channels over coaxial cables, power lines, or via radio frequency (RF) transmissions, degrades the signals significantly and results in relatively low-quality audio and video output.
Although some systems utilize existing unshielded twisted pair (UTP) telephone wiring to carry analog audio and video signals, the analog modulation of source audio and video signals over UTP cables also produces relatively low-quality audio and video output. Moreover, such systems still require custom installation of such UTP cabling. As a result, the expense and complexity of such systems increases exponentially compared with single-room home theater systems.
Local Area Networks and the Ethernet
One potential solution to the problems noted above is to implement a digital computer-based network of the type typically found in business environments for inter-connecting personal computers, workstations, servers, and printers. Although the recent popularity of the home computer market has led to a great deal of discussion of home networks, which merge general-purpose computing functionality with the consumer electronics devices found in home theaters, the solutions offered thus far have not advanced beyond the proposal stage.
Applying local area network technology (e.g., the Ethernet network protocol and the transmission control protocol/Internet protocol {TCP/IP}) to consumer electronics devices raises a number of problems. Although it appears advantageous to connect consumer electronics devices as generic nodes on a network, existing network protocols are not optimized for real-time streams of digital audio and video.
Conventional “solutions” typically fall into one of two categories. The first is analogous to the home automation networks discussed above, in which consumer electronics devices are connected via dedicated audio and video cables, and still transmit analog information along one network, while an Ethernet network, for example, enables home computers and other control devices to control the operation of the consumer electronics devices.
The other alternative implementation is to distribute audio, telephone, video, and other real-time media streams in digital form. There are a number of obstacles to this scenario, however, not the least of which is the absence of an existing physical and logical infrastructure to carry the digital media streams. The Ethernet, for example, is not optimized to carry real-time continuous digital media streams. It is an asynchronous, packet-based protocol that would add significant overhead to digital audio and video samples, which require consistent and timely delivery, as opposed to the ability to send “burst” packets of information at high speeds on demand.
Several known standard audio transfer file formats attempt to synchronize audio streams as used in the field of stereo equipment. For example, S/PDIF (Sony/Phillips Digital Interface) is typically used with digital audio equipment to enable the transfer of audio (e.g., over coaxial cable) from one file to another and the synchronization of audio streams. Also, TOSLINK (Toshiba Link) allows the synchronization of audio streams over fiber optic cables for stereo equipment. However, both are unsatisfactory solutions as they provide unidirectional data flow, and are unable to handle control signal transmissions.
Synchronous Networks
Synchronous network protocols (as discussed below) have not been optimized or adapted for popular use with consumer electronics devices to enable the practical distribution of digital media. For example, time-division multiplexed access (TDMA) networks utilize time-division multiplexing, and synchronize all devices to a master clock. However, the bandwidth on a TDMA network is typically divided equally amongst the devices on the network. In other words, if ten devices are on the network, each device gets one tenth of the network bandwidth, and thus can transmit information only during that channel or “time slice” (e.g., during one unit of every ten units of time). While some TDMA systems can allocate multiple time slots for higher bandwidth devices, TDMA is generally an insufficient alternative because in the context of transmitting digital media streams, certain data streams require more bandwidth than others. For example, video data requires more data than audio data, though sampled less frequently. Yet, TDMA networks assign each device a single channel in which to transmit all of its data. These channels are based simply on the number of devices on the network, and bear no relationship to the bandwidth requirements of the type of data being transmitted. This problem is exacerbated when asynchronously distributed variable bit-rate data, such as MPEG2 compressed video, needs to be accommodated. TDMA network technology provides no solution to either of these problems.
Even in the context of a ring network, in which data propagates from one device to another around a loop or ring (and is overwritten when a device desires to insert its own data), a device on a TDMA network could transmit information (such as a digital audio sample) anytime during its assigned time slice. Moreover, that time slice might change whenever a new device is added to or removed from the network. Thus, a device cannot guarantee consistent delivery of particular data, despite the synchronous nature of TDMA.
Fiber distributed data interface (FDDI) networks transmit information synchronously only in a point-to-point manner. In other words, the transmitter on one device is synchronized to the receiver on the next device on the ring, but the transmitter and receiver within a device are not synchronized to each other. Therefore, information will not always propagate through a device at a consistent rate, due to the difference between the transmit oscillator and receive oscillator within a device, among other factors. FDDI devices compensate for this difference with an “elasticity buffer” which avoids losing data, but this does not guarantee consistent delivery of data. For example, if a device receives data “late,” it will transmit that data late. If it receives data “early,” it will place that data in its elasticity buffer, and transmit such data in a first-in-first-out (FIFO) fashion. Thus, FDDI devices also cannot guarantee consistent delivery of data such as real-time continuous digital media streams. They are optimized for high throughput, but not for consistent, synchronous delivery of data. When FDDI is implemented using a LAN system (e.g., token ring access methods), FDDI is operable when a loop exits. Since breaking the loop interrupts the network, this technique of using FDDI falls short of providing a synchronous network.
Furthermore, both TDMA and FDDI systems do not transmit bi-directional data over a single pair of wires. This means that multiple pairs of wire must be run between devices, or the network must always be wired in a physical loop configuration.
Additionally, the IEEE 1394 high-speed serial bus works well with USB by providing enhanced PC connectivity for a variety of devices, including consumer electronics audio/video (A/V) appliances, storage peripherals, other PCs, and portable devices. Several examples of the IEEE 1394 standard include FireWire from Apple Computer, and iLink from the Sony Corporation. Although many peripheral devices, as well as stereos and televisions, have adopted this standard for communication over bi-directional high speed networks including access to CE equipment (devices), these IEEE 1394 implementations nevertheless fail to include the transmission of time synchronization signals. This is problematic as synchronization issues are left to the individual devices.
The above description of home theater systems and home automation networks, and of various existing asynchronous and synchronous network protocols, illustrates many of the obstacles to interconnecting consumer electronics devices for distribution of audio, telephone, video, and other real-time continuous digital media streams throughout a home or other environment. These obstacles must be resolved before home networks can achieve widespread acceptance. What is needed is a network that can accommodate real-time continuous digital media streams (e.g., digitized audio, video, and telephone). To do so, the network should deliver digital media streams reliably with a high quality of service, in order to provide the same level of synchronization as is currently provided by existing analog delivery mechanisms.